1. Field of the Invention
The present invention relates to acidizing processes for treatment of subterranean reservoir formations and, more particularly, relates to acidizing processes for treatment of asphaltenic reservoirs.
More particularly, the present invention relates to novel dithiocarbamate compositions which are particularly effective as aids in preventing asphaltene precipitation in asphaltenic reservoirs treated with a solution of a strong acid, and to methods of use of such novel compositions.
Asphaltenic crudes contain aromatic-based hydrocarbons of amorphous structure, generally referred to as asphaltenes. These asphaltenes are present in crude oil in the form of a colloidal dispersion which consists of an aggregate of high molecular weight polyaromatic molecules surrounded by lower molecular weight neutral resins in paraffinic hydrocarbons. This colloidal dispersion is stabilized by an absorbed layer which may be less aromatic and more paraffinic in nature. A negative surface charge has been postulated based on charge neutralization studies. Thus, highly charged positive species such as acid protons can have very dramatic impact on the surface charge by neutralizing it and allowing larger aggregates of these colloidal particles to form. The larger aggregates then tend to precipitate thereby forming sludges and/or rigid-film emulsions.
Various acid solutions have long been used to increase the permeability of a formation surrounding a well bore. In these treatments, the acid is normally pumped under pressure from the surface, through iron pipe and into the formation to dissolve various portions thereof. Such acid treatments have been particularly effective in calcareous formations and have been effective in increasing the production of oil, gas, brine or even water wells.
When these acid solutions are utilized to treat asphaltenic reservoirs, asphaltenic sludge and/or rigid-film emulsions develop and formation damage results in and around the well-bore area. As a consequence the well may be very slow to clean up or oil flow may be completely inhibited.
Anti-sludging agents, such as dodecylbenzenesulfonic acid, are typically used to control the formation of asphaltenic sludges caused by the addition of acid to asphaltenic reservoirs. Such anti-sludging agents are typically dispersed within the acid before treatment of the reservoir. However, such anti-sludging agents are significantly less effective in the presence of ferrous iron.
It is well documented that the use of acid solutions in reservoirs typically dissolves significant portions of ferric and ferrous iron from containers or conduits or components of subterranean reservoir formations. In addition, the acids utilized are typically contaminated with iron. Although asphaltenic sludges are more readily formed in the presence of ferric iron, relative to ferrous iron, tests have indicated that the ratio of ferrous to ferric iron ranges from between 5:1 to 10:1. The high levels of ferrous iron can be explained on the basis of a strong driving force for converting ferric iron to ferrous iron while the acid is being pumped because such acid is in contact with iron pipe. The net reaction is retarded by the presence of oxygen. The presence of such ferrous iron increases asphaltene precipitation and inhibits the effectiveness of anti-sludging agents.
Thus, although acidizing procedures are, in general, effective in increasing the production of oil by increasing the permeability of a formation surrounding the well bore, such procedures cause a variety of problems when used to treat asphaltenic reservoirs, many of which are due to the presence of ferrous iron.
Thus, by sequestering ferrous iron present in the acidic solutions to form iron complexes which are dispersible in highly acidic media and which are soluble in asphaltenic crudes, the effectiveness of the anti-sludging agents will be greatly increased thereby eliminating the problems associated with acidizing asphaltenic reservoirs. Furthermore, by reducing the ferric iron present in the system to ferrous iron, or by reducing the amount of oxygen in the system, the adverse effects of ferric iron can be simultaneously reduced or eliminated.
2. Prior Art
The most common iron sequestering agents in commercial practice are citric acid, ethylene diamine tetracetic acid (EDTA), nitrilo triacetic acid, acetic acid, lactic acid and citric acid/acetic acid mixtures. Other iron sequestering agents include ascorbic acid, erythorbic acid and salts thereof which are disclosed in U.S. Pat. No. 4,574,050 and which are utilized to prevent precipitation of ferric compounds from spent acid solutions. U.S. Pat. No. 4,096,914 discloses the use of salicylic acid to chelate the ferric ions that are dissolved within the acid and U.S. Pat. No. 4,137,972 discloses the use of sulfosalicylic acid for the same purpose. However, all of such iron sequestrants are only effective in acidic solutions wherein the pH is above about 2, and therefore are ineffective in acid solutions utilized in acidizing reservoirs wherein the acid solutions are typically from about 15% to 28% acid.
Thus, prior art efforts have not been directed at sequestering ferrous iron at very low pH to produce iron complexes which are dispersible in highly acidic media and which are soluble in asphaltenic crudes. Furthermore, such efforts have not been directed at preventing ferrous iron-asphaltene interaction to thereby permit anti-sludging agents to be effective.